A comet's volatiles won't evaporate as long as it remains in the outer Solar System. But every so often, one of them gets perturbed into an orbit that makes it visit the inner Solar System regularly. The comet will then do a lot of evaporation and will eventually disintegrate, likely in a time much less than the age of the Solar System. Someone might have tried to estimate how long a typical comet will last; I'd have to search the literature for that.
The Rosetta probe has woken up ...
- Thread starter GlennB
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A comet -- when it's inactive at least -- is very close to thermal equilibrium. The region in which they are thought to have formed was enriched by volatile ices which were evaporated from the inner solar system by the sun's heat and transported to the outer solar system by the solar wind where they were at a temperature where they could condense into solids and be incorporated into larger bodies though collapse and collisions. Some of these objects that formed on the edges of the system had their orbits altered in a way that sent them into the inner solar system where again the temperatures causes them to evaporate and be pushed away from the sun by the solar wind.
There is a question as to why smaller, short-period comets (like Halley's Comet with a period of 75 years) still have an active nucleus (i.e. they are still shedding large amounts of volatiles when they are in the inner solar system) -- why haven't they exhausted their supply long ago.
Of course, they probably didn't always exist as a short-period comet and instead may have been, for instance, a perfectly happy Kuiper Belt Object until they crashed into something else or got to close to a massive object, etc.. So their active period would not have not been the whole 4 billion years.
The object itself didn't condense/accrete out of pure volatile ices. They are a dirty snowball meaning they have a significant mass of non-volatile things like silicates -- which are fine with taking a close-in trip around the sun. So when you think about the surface of the comet as it enters the active phase you are preferentially losing the volatiles -- the evaporated and are driven away by solar wind leaving behind the dust. The next generation that is exposed does the same. So the active nucleus can become kind of corked underneath a layer of inactive material for some period of time. This is one of the reasons it's thought the apparent brightness of new and long-period comets is so hard to predict. This could prolong the life of an active nucleus until some future event where the cork is popped and the volatiles again are exposed.
An astrophysicist who was asked this question on the 'Naked Scientist' program estimated 10,000 years as the average lifespan of a comet.
http://www.thenakedscientists.com/HTML/questions/question/1000112/
That is, of course, once it has gotten into an orbit that leads it close to the sun periodically.
She is also quoted as saying (similar to what you said above) , that the object will eventually completely sublimate.
My understanding so far was that once all the volatiles are gone comets will essentially resemble asteroids (i.e. extinct comets). Maybe this would be only true for some comets?
Gawdzilla Sama
TImeToSweepTheLeg
"Even the weariest comet
eventually winds to the Sun."
eventually winds to the Sun."
Or she was not being precise enough -- the rocky parts of comets won't sublimate.
BTW, in another messageboard, someone claimed that the enrichment of a certain dust grain in sodium was due to sublimation. The original results: COSIMA detects sodium and magnesium in a dust grain called Boris | Rosetta - ESA's comet chaser COSIMA measured Na-23+ and Mg-24+ and found
(Na)/(Mg) peak heights = 4.6
This is unusual enrichment in sodium by Solar-System standards.
The bulk composition of rocky materials in the Solar System, as far as it can be inferred, is mafic or basaltic, enriched in magnesium and iron. In the Sun and some meteorites,
(Na)/(Mg) atoms = 0.05 - 0.06
The Earth's continental crust is enriched in sodium because of fractionation of magmas, making it felsic or granitic.
(Na)/(Mg) atoms = 0.75 to 2.0 (estimates jump around quite a bit)
But Boris is even more enriched in sodium.
This cannot be due to sublimation at 67P's getting as close as it does to the Sun, because rocky materials are generally very refractory. One would have to go *very* close the Sun to make them start to boil off.
So this grain either formed when the Solar System formed or else was a grain from some other star that found its way into the protosolar nebula.
The Rosetta team ought to look for more elements, and also analyze more grains, but they might only release their results when they are ready to publish them.
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Philae's rocky road to 'a bit of a bounce'.
Lovely mosaic of its descent and dusty footprint upon hitting the comet.
"It [a sensor] retrieved a number of temperature profiles but broke as it tried to burrow its way into the comet's subsurface. Scientists say this shows the icy material underlying 67P's dust covering to be far harder than anyone anticipated - having the tensile strength of some rocks. It also helps explain why Philae bounced so high on that first touchdown."
Lovely mosaic of its descent and dusty footprint upon hitting the comet.
"It [a sensor] retrieved a number of temperature profiles but broke as it tried to burrow its way into the comet's subsurface. Scientists say this shows the icy material underlying 67P's dust covering to be far harder than anyone anticipated - having the tensile strength of some rocks. It also helps explain why Philae bounced so high on that first touchdown."
davefoc
Philosopher
Why would they expect CO2 and H2O frozen to -70 C to not be really hard?
The internal structure of the comet continues to interest me. Supposedly the density of the comet is about 40% that of water. So two possibilities: 1. The comet is a spongy matrix with more than half the volume being empty or 2. the comet contains a lot of frozen gases with densities much less than CO2 and H2O. If it is possibility one which seems to be the current view how does the dust which presumably makes up the matrix have enough strength to sustain the structure. Perhaps the dust and frozen gases make up a compound material that's strong enough?
I wonder if what is going on here is that the dust and gases randomly stick to the surface of a forming comet. The chaotic nature of the process leaves voids which the gravity of the comet is not strong enough to crush.
Is this the first time that this comet has passed by the sun? Will the comet change radically after it has been out gassing for awhile?
BenBurch
Gatekeeper of The Left
I think they were expecting something like a new york city snow bank - packed hard with lots of dirt inclusions but not strongly welded together.
I suspect what happened is that they didn't anticipate the degree of freezer burn.
davefoc
Philosopher
This might have been obvious to most people but I haven't been able to come up with an idea that seemed quite right about the nature of the comet. I think now that the interior of a comet structurally might be like a tub of ice cubes that has melted a bit and then become refrozen. This kind of structure can be fairly strong, certainly strong enough to hold up to .000001 g. Assuming I'm right about this I still have questions:
1. How does the fact that the structure is made up principally of two different frozen gases affect things?
2. How does the fact that dust is mixed in effect things?
3. What is the chemical composition of the dust?
4. How big are the grains of ice that constitute the comet? Maybe they're tiny like the grains of ice in a freezer that needs defrosting or maybe they're much larger.
5. How did this structure come about. Two ideas:
A. The structure happened as gases froze to the surface of the comet. They preferentially froze to some places, (freezing nucleation sites?) rather than freezing into a homogeneous sphere.
B. Something roughened the surface as the gas froze evenly on the surface.
I notice that when water vapor solidifies inside a freezer it freezes into something that resembles stuck together snow. Maybe this is a pretty good analog for how the comet forms.
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ESA has released a sound recording of the lander's first touchdown:
http://blogs.esa.int/rosetta/2014/11/20/the-sound-of-touchdown/
Here is some information about the surface of the comet at the location of the first touchdown:
In other news, there is some speculation at ESA that Philae might be sheltered enough to survive until the comet reaches perihelion. That would be very exciting.
http://blogs.esa.int/rosetta/2014/11/20/the-sound-of-touchdown/
Here is some information about the surface of the comet at the location of the first touchdown:
In other news, there is some speculation at ESA that Philae might be sheltered enough to survive until the comet reaches perihelion. That would be very exciting.
Southwind17
Philosopher
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How did they ensure that Philae landed feet first, especially after the first bounce?
rwguinn
Penultimate Amazing
Look up "Gyroscope" in your google...
phunk
Illuminator
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It had a reaction wheel to maintain orientation until the landing. But it was powered off automatically at the first contact with the ground, so the spacecraft started to turn on its own at that point, and it was just luck that it didn't land upside down on the next landing (although it didn't land on all of its feet either).
davefoc
Philosopher
I had been wondering the same thing.
I looked around for information on reaction wheels. This is an article by one of the engineers that worked on the Philae reaction motor and perhaps the Rosetta reaction motors.
http://www.sstl.co.uk/Blog/November-2014/How-a-small-wheel-helped-to-stabilise-Philae
Unfortunately his article was weak on detail about what was going on. Wikipedia had a general article about reactions wheels that was more informative technically:
https://en.wikipedia.org/wiki/Reaction_wheel
The Rosetta main spacecraft had four reaction wheels and one of them failed. It sounds like the reason that Rosetta had four was to provide redundancy. To completely control the spacecraft's attitude three are needed pointed perpendicular to each other. By using four positioned to point perpendicular to the points of a tetrahedron it sounds like one can fail and with the right software the attitude of a spacecraft can be controlled.
However, I think there may have only been one on Philae so it didn't function as a reaction wheel. The reaction wheel's engineer said it was a momentum wheel which functions to make a spacecraft more resistant to changes in attitude but can't be used to adjust it like reaction wheels can. This makes it sound to me that it wouldn't have helped to keep the momentum wheel spinning after the first touch down because Philae would have some random attitude and the momentum wheel couldn't correct it.
davefoc
Philosopher
Another one of the things I've been wondering about is how H20 forms in space. The model I imagined was of oxygen getting blasted out of a star at end of life that contained oxygen created by fusion within the star.
So I imagined cold oxygen and hydrogen traveling through space in an environment where it was too cold initiate the reaction that produces water.
Based on what I read it sounds like there are three paths to the formation of water in space. One happens at temperatures above 300K where there is enough thermal energy to initiate the reaction. When the temperature is below 300K there are two other kinds of reaction that can produce water. From the article that I found discussing this:
Gas-phase
Regardless, the paper I found claims that almost all the oxygen in space is bound up mostly in water ice and that almost all the water in space is in the form of ice.
Link to the article about this:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCAQFjAA&url=https%3A%2F%2Fwww.sofia.usra.edu%2FScience%2Fscience_cases%2FBergin2008.pdf&ei=WWFyVPmBG4TsoATc-oDgCw&usg=AFQjCNF9DuHhCitj1U8ipz4Vv1QZ03reqQ&sig2=rQcFrxJnDXrFJzRZDY66uQ&bvm=bv.80185997,d.cGU&cad=rja
So I imagined cold oxygen and hydrogen traveling through space in an environment where it was too cold initiate the reaction that produces water.
Based on what I read it sounds like there are three paths to the formation of water in space. One happens at temperatures above 300K where there is enough thermal energy to initiate the reaction. When the temperature is below 300K there are two other kinds of reaction that can produce water. From the article that I found discussing this:
Gas-phase
It's interesting that these dust grains can act as a kind of catalyst for the formation of water. I don't understand what is going on there at all. Does the reaction stop once the dust grain is covered in water ice?
Regardless, the paper I found claims that almost all the oxygen in space is bound up mostly in water ice and that almost all the water in space is in the form of ice.
Link to the article about this:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCAQFjAA&url=https%3A%2F%2Fwww.sofia.usra.edu%2FScience%2Fscience_cases%2FBergin2008.pdf&ei=WWFyVPmBG4TsoATc-oDgCw&usg=AFQjCNF9DuHhCitj1U8ipz4Vv1QZ03reqQ&sig2=rQcFrxJnDXrFJzRZDY66uQ&bvm=bv.80185997,d.cGU&cad=rja
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That sounds right. You might remember the Kepler spacecraft. It has four reaction wheels and could be kept fully functional after one failed. When the second one failed, Kepler could no longer be aimed accurately and the primary mission ended.
davefoc
Philosopher
I understood this to some degree, but my idea was that the oxygen would not combine with the hydrogen as it was spewed from the star because it would be too hot initially and by the time it had cooled to a temperature where it might combine the concentration of hydrogen and oxygen would be too low. Water almost completely disassociates at around 3500K. So for water to form, the temperature that it is spewed out needs to drop below 3500K before the concentration of hydrogen and oxygen is to low for a significant amount of water formation to occur. Eventually the temperature of the hydrogen and oxygen will drop below 300K and there will not be enough energy available to initiate the combination of hydrogen and oxygen into water.
It seems that there is some water created during this initial cooling process but there is still a lot of elemental oxygen that makes its way into space. That's when the two other mechanisms mentioned in the article come into play. Combination facilitated by dust and combination facilitated by cosmic rays.
caveat:Obviously I'm not a chemist or physicist and I may not have everything exactly right above.
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davefoc
Philosopher
This also explains what was going on with Hubble when it needed to have some of its reaction wheels changed although at the time I remember them being identified as gyroscopes. It sounds like reaction wheels are a kind of gyroscope where the spin rate is varied to control the spacecraft attitude.
I'm surprised reaction wheels are so unreliable. Electric motors are just about the most reliable electromechanical devices on the planet. Electric refrigerators run for generations and I doubt that people pour as much effort into making them reliable as they do spacecraft reaction wheels. Still most refrigerators aren't blasted into space and exposed to multiple g loads and then operated across the wide temperature variations that a spacecraft experiences and it may be that they need to operate in a vacuum as well. Maybe I should cut the reaction wheel engineers a little slack.